This invention relates to free piston Stirling engines, and more particularly to a sealed oil-backed diaphragm for suspending a free displacer in a free piston Stirling engine.
This invention is related to application Ser. No. 172,373 for "Diaphragm Displacer Stirling Engine Powered Alternator-Compressor," filed on July 25, 1980, by Folsom, et al., and to application Ser. No. 270,974, "Oil Backed Displacer Diaphragm," filed by Jeffrey S. Rauch concurrently herewith, the disclosures of which are incorporated herein by reference. The engine of the '373 application is a free piston Stirling engine which utilizes a diaphragm to suspend the displacer in the working space and uses the pressure wave in the working space to maintain the displacer oscillation. Although this machine, and the improvement disclosed in the 270,974 application constitute a significant improvement in the art, there are some areas in which modifications would improve their operation.
In the machine of the '373 application, the displacer diaphragm is subjected to stress induced by the pressure swing of the working gas in the working space which is on the order of 10%-20% of the charge pressure in the working space which can be on the order of 40-80 bar. Therefore, the pressure swing can be on the order of 4-8 bar which, acting over the full face of the diaphragm, can introduce considerable stress in the diaphragm. This complicates the deformation pattern of the diaphragm and reduces its working life. This pressure induced stress does not contribute to the operation of the machine. The only stress that is desirable as a design function is displacement induced stress, that is, the spring effect contributed by the diaphragm when it is displaced from its central position. This necessary and desirable stress in the diaphragm is compounded and multiplied in many ways and with deleterious results by the pressure induced stresses in the diaphragm so that the diaphragm design is greatly complicated and diaphragm reliability and repeatability is decreased. Moreover, the effect changes with pressure and therefore an additional degree of difficulty is encountered if a power control system based on mean pressure variation is introduced.
A second area of improvement which would be desirable is control of the power input into the displacer itself. Power input into the displacer is related to the ratio (.DELTA.V/V), where .DELTA.V is the difference in the volumetric displacement of the displacer in the expansion space and the compression space, and V is the volumetric displacement of the expansion space. The power required to maintain the oscillation of the displacer in the working space, that is, to overcome the friction and windage losses of the working gas in the heat exchangers, normally requires a (.DELTA.V/V) ratio of approxmately 0.1. However, the value of approximately 0.3 is normal for a diaphragm in an engine of this variety. This provides more energy to the displacer than it needs and thus causes the displacer to slam back and forth between its stops unless some means is provided to extract the excess energy put into the diaphragm by the thermodynamic system. Alternatively, some technique must be provided for limiting the (.DELTA.V/V) ratio to a value more suited to the engine operation.
Along these lines, it is possible by artful design of a displacer to enable it to operate with the desired characteristic, that is, with a small (.DELTA.V/V). However, while the diaphragm is physically capable of operating in this manner, it is also capable of other patterns of displacement and there is no assurance that it will indeed perform in the desired manner when operated in the engine at various pressures and other operating parameters. Therefore, it is necessary to impose some form of restraint on the deformation pattern of the diaphragm in its operation so that it will conform to the desired configuration.
Another possible improvement that should enhance the diaphragm life and reliability in a reduction of the stress which the diaphragm must carry. The diaphragm is a mechanical suspension member that acts also as a spring to center the displacer and return it toward the hot end of the working space after its displacement toward the cold end by the pressure wave. The stress level in the diaphragm approaches the maximum allowed at the displacer stroke extremes. At these extremes, it would be desirable to have a supplementary spring member assume a portion of the load so that the diaphragm need not carry the entire load.